The present invention relates to a method for controlling an exhaust gas recirculation (EGR) system of an engine mounted on a flexible fuel vehicle (FFV), and more particularly to a control method for properly reducing the amount of NOx which included in exhaust gases.
Various attempts have been made to provide engine control systems enabling the engine to be operated with alcohol mixed fuel because of energy shortage and improvement of emission control in these days.
The flexible fuel vehicle having such an engine control system can be driven with gasoline only, or mixed fuel of gasoline and alcohol, or alcohol only. Namely, the concentration of the alcohol in the fuel changes between 0% (gasoline only) to 100% (alcohol only) in dependency on situation of a driver of the vehicle.
An alcohol concentration sensor is provided in a fuel system for sensing the concentration of alcohol in the fuel. An alcohol concentration signal is applied to a control unit in which fuel injection quantity, ignition timing, and desired supercharging pressure are properly determined in dependency on the alcohol concentration, thereby properly operating the engine.
Alcohol has a lower combustion temperature than that of gasoline. Therefore, if the alcohol concentration of the fuel is high, the amount of NOx in the exhaust gases is considerably reduced. Since an octane number of the fuel is increased under above described condition, it is possible to set a compression ratio of the engine to a high value for the FFV, thereby improving thermal efficiency. However, if the engine has a high compression ratio, the combustion temperature rises to cause the amount of Nox to increase when fuel having a high gasoline concentration is used. In order to effectively reduce the amount of NOx, it is desirable to use the EGR system in the engine for the FFV.
Japanese Patent Application Laid-open 56-162256 discloses an EGR system of an engine for the FFV. If combustion efficiency decreases in a high alcohol concentration range, the operation of the EGR system is stopped or the amount of recirculating exhaust gas is reduced, thereby improving the engine efficiency.
However, the amount of recirculated gas is only controlled at two values in accordance with the alcohol concentration, so that the EGR system can not be properly controlled under various operating conditions of the engine. It is necessary to finely control the EGR system in consideration of the alcohol concentration, the engine operating conditions, output power of the engine, and fuel consumption.